1. Field of the Invention
The invention relates to radio frequency (RF) security labels having a deactivatable resonant circuit.
2. Description of the Related Art
Electronic article surveillance (EAS) systems are widely used in commercial and retail establishments to deter theft or other unauthorized removal of articles from the surveillance area. In one commonly used EAS system, each article or item to be protected carries a security tag or label, which may be affixed on or inside packaging or as a label for the article or item, or on or inside the article or item itself, containing an electronic circuit, such as an inductor/capacitor resonant circuit. The resonant tag circuit is detected or identified by equipment for establishing an RF electromagnetic field in a surveillance zone at the exit of the surveillance area.
The detectable resonant circuit of the EAS tag is a small, generally planar, multi-layer structure having a dielectric substrate and conductive layers on opposite sides of the substrate that define an inductor and at least one capacitor that provide a circuit resonant at at least one predetermined detection frequency.
Removal of a tagged article from the surveillance area is typically authorized at a checkout counter, where the clerk deactivates the tag. The security tag may be deactivated by changing the resonant frequency of the tag so that the tag resonates outside of the predetermined detection frequency or by altering the resonant circuit so that the circuit no longer resonates.
A typical deactivation technique is accomplished electronically, by passing the tag through a deactivating RF field that disables the detectable resonant circuit. Such deactivation involves exposing the resonant tag circuit to an RF field having a predetermined minimum energy level sufficient to cause either short-circuiting of the resonant circuit or creation of an open circuit and thereby preventing the circuit from resonating at the predetermined detection frequency.
In a typical implementation of deactivation a portion of one conductor in a multi-layer resonant tag circuit is indented or “dimpled”. Deactivation is accomplished by exposure of the tag to a specific RF field at a predetermined energy level that causes a short circuit at the indent or dimple, which results in the desired deactivation of the resonant circuit being targeted. Examples of “dimpled” deactivation tag devices are shown in U.S. Pat. Nos. 4,567,473 and 5,841,350. Other deactivation techniques include the use of fuseable links as shown in U.S. Pat. Nos. 4,802,944 and 5,059,950.